The present invention relates to resistance welding and, more particularly, to an improved electrode for use in a resistance welding device.
Resistance welding has long been used as a quick and effective method of joining metal members. The workpieces to be welded are placed in an abutting relationship and a large current is caused to flow across the boundary between the members by a pair of opposed electrodes. The current causes the surfaces of the workpieces to be heated sufficiently to cause the formation of a weld nugget. Typically, the electrodes apply significant pressure to the workpiece during welding. This facilitates the welding process by urging the material together and, also, reducing electrical resistance between the electrode tip and the workpiece material.
Since the welding is accomplished by resistance heating of the material being welded, it will be appreciated that the electrodes will also be heated substantially. It is important to have an electrode of high electrical conductivity in order to minimize the power loss in the electrode and the resulting heating of the electrode. It is also important to have an electrode which is capable of withstanding significant distorting force at the elevated temperatures which result from the welding process. Hollow electrodes have long been used for resistance welding with cooling fluid supplied to the interior cavity in order to reduce substantially the temperature of the electrode shank. It will be appreciated, however, that this method of cooling has certain limitations and, further, that the electrode tip cannot be cooled effectively in this manner.
Electrodes have, in the past, been formed of high conductivity copper in order to minimize the power loss in the electrodes. Such electrode material has a relatively limited life, however, which is due in large part to deformation of the electrode tip after repeated welding operations at high temperature and pressure. It has been the usual practice to reshape or redress the electrode tips to the desired shape. This can be accomplished only a limited number of times, however, and eventually the electrode must be discarded. Not only is it expensive to discard such electrodes, but the down time of the welding machine for replacement of redressed electrodes may be even more expensive.
In order to minimize the cost of scrapping copper electrodes, two piece electrodes having a replaceable electrode tip and a reusable shank have been used. U.S. Pat. No. 2,440,463, issued Apr. 27, 1948, to Cornwall, U.S. Pat. No. 2,780,718, issued Feb. 5, 1957, to Mullen, U.S. Pat. No. 2,829,239, issued Apr. 1, 1958, to Boretti, and U.S. Pat. No. 2,402,646, issued June 25, 1946, to Leathers, all show replaceable electrode tips which are frictionally engaged by a shank portion of the electrode. U.S. Pat. No. 2,437,740, issued Mar. 16, 1948, to Haynes, and U.S. Pat. No. 2,472,173, issued June 7, 1949, to Powell, show mechanical brackets or set screw arrangements for holding the replaceable welding electrode tip.
Such a replaceable tip may also be attached to the shank portion by threading engagement as shown in U.S. Pat. No. 2,761,973, issued Sept. 4, 1956, to Kerr, U.S. Pat. No. 2,796,514, issued June 18, 1957, to Wood, and U.S. Pat. No. 3,310,087, issued Oct. 29, 1963, to Larkworthy. Both U.S. Pat. No. 2,257,566, issued Sept. 30, 1941, to Lewis and U.S. Pat. No. 2,411,859, issued Dec. 3, 1946, to Harwood, show welding electrode tips or tip portions which are mechanically interlocked with a shank portion. In the Harwood device, a reinforcing cap of hardened metal surrounds but does not cover the electrode tip. In the electrode of Lewis, a replaceable tip is pressed into interlocking engagement with the shank portion.
In U.S. Pat. No. 3,446,932, issued Aug. 10, 1948, to Johnson, a replaceable tip for a spot welding electrode is disclosed which is formed from a hardened material, e.g., a drawn copper slug. The tip is then bonded to the electrode body by fusible material, such solder, which has a fusion point lower than the annealing temperature of the tip. U.S. Pat. No. 2,138,388, issued Nov. 29, 1938, to Platz, discloses a replaceable electrode tip which is welded to the shank. U.S. Pat. No. 2,795,688, issued June 11, 1957, to McCaffrey, discloses a welding electrode having a stainless steel alloy tip which is brazed onto a shank made of copper.
U.S. Pat. No. 3,909,581, issued Sept. 30, 1975, to Stone et al, discloses a composite resistance welding electrode having a holder made of an inexpensive, relatively soft metal with high electrical and thermal conductivity and a tip which has additional strength at resistance welding temperatures. The tip may be formed of a more costly material, such as various copper alloys. The tip may be connected to the shank portion with a pressure fit or, alternatively, by brazing. A pressure fit will generally be unacceptable due to the high electrical resistance at the joint. If the tip is brazed onto the shank, however, the shank may be somewhat annealed and weakened. Thus, the improved number of welding operations which could be expected from such an electrode are reduced.
One material which has recently been developed and which has shown high promise for use in resistance welding electrodes is a dispersion strengthened copper which is formed by internal oxidation of a dilute copper-aluminum alloy. This material is extremely hard at welding temperatures and highly conductive. U.S. Pat. No. 3,779,714, issued Dec. 18, 1973, to Nadkarni et al, U.S. Pat. No. 3,884,676, issued May 20, 1975, to Nadkarni et al, and U.S. Pat. No. 3,893,841, issued July 8, 1975, to Nadkarni et al, disclose dispersion strengthened metals of the type intended to be used with the present invention. As discussed in the August 1976 edition of METALS ENGINEERING QUARTERLY, pages 10-15, in an article by Nadkarni et al, this dispersion strengthened copper alloy material produces superior welding electrodes.
U.S. Pat. No. 4,045,644, issued Aug. 30, 1977, to Shafer et al discloses a welding electrode which is formed completely of a dispersion strengthened copper material. The electrode is produced by pressure flowing a blank transversely of an axially applied pressure. One problem with producing such an electrode in this manner, however, is that a relatively long, slender electrode cannot be formed, since the grain structure which would result produces an electrode of less than maximum strength. Another disadvantage of such a solid, dispersion strengthened copper electrode is the relatively high cost; this dispersion strengthened copper alloy material is more than twice as expensive as a conventional chrome copper alloy.
U.S. Pat. No. 3,969,156, issued July 13, 1976, to Wallbaum, discloses a method of making a welding electrode having a portion of the electrode formed of a dispersion strengthened copper material. In the Wallbaum method, discs of dispersion strengthened copper are interspersed in a stack with discs of high conductivity copper. The stack of discs is then extruded to form the resistance welding electrode. Following the extrusion process, an extra step is required in which the electrode is heat-treated for precipitation hardening of the high conductivity copper portion of the electrode.
In U.S. Pat. No. 3,184,835, issued May 25, 1965, to Coxe et al, a bi-metal welding electrode is disclosed having an internally oxidation hardened alloy tip. The tip is brazed to a shank after the tip and shank are formed. The disadvantage in such a process, as discussed with respect to the Stone et al patent above, is that the shank portion of the electrode may become annealed during the brazing operation, resulting in a relatively weak electrode structure.
Accordingly, it is seen that there is a need for a bimetal electrode having a dispersion strengthened copper tip and a shank portion formed of a less expensive, high conductivity copper which sufficiently hard to withstand the stress of a resistance welding operation.